The performance of an acoustic transducer such as an audio loudspeaker is greatly influenced by nearby acoustic boundaries, such as ceilings, floors, and walls. The acoustic waves reflecting from these boundaries are sometimes in phase, sometimes out of phase with the acoustic waves coming directly from the speaker. This results in variations in the acoustic power output of the speaker as a function of frequency and also results in irregularities in the sound pressure as a function of frequency.
For frequencies whose wavelengths are long compared with the distance from the speaker diaphragm to an acoustic reflecting boundary, the reflected sound is substantially in phase with the direct sound from the diaphragm of the speaker or other acoustic transducer. If the mechanical impedance of the speaker is much greater than the radiation resistance load on the diaphragm, then the reflected sound will produce increased sound pressure levels and therefore greater power output from the acoustic transducer or speaker. Thus the low frequency power output is increased by approximately 3db by one area of a nearly acoustical boundary, 6db by two nearly mutually perpendicular areas of an acoustical boundary and 9db by three nearby mutually perpendicular areas of an acoustical boundary. In contrast the power output is affected very little by acoustical boundaries at high frequencies where the wavelengths are less than twice the distance from the diaphragm of the acoustic transducer to the acoustic boundary. At intermediate frequencies where the wavelengths are approximately two to five times the distance from the diaphragm of the acoustic transducer to the reflective acoustic boundary there is less acoustic output than if there were no reflecting boundary because of destructive interference between direct and reflected waves. The severity of this reduction of the power output of the acoustic transducer increases with the number of areas of which the acoustic reflective boundary is made: an acoustical boundary having three areas each equidistant from the diaphragm of the acoustic transducer or speaker produces a much larger dip in the response than a boundary having but one such area.
There are important similarities between the effect of nearby boundaries on power response and the effect of nearby boundaries on pressure response and on excitation of room standing wave modes. For low frequencies, the sound pressure increases due to the proximity of a boundary, the increase becoming greater as the number of areas of the boundary increases. However, in the frequency range where the wavelengths are approximately two to five times the distance from the diaphragm to the boundary, the sound pressure is less than if there were no boundaries. Similarly if a transducer is much closer to one boundary of a pair of parallel boundaries, then all low frequency standing wave modes are excited, whereas in approximately the same intermediate frequency range described above, the standing wave modes are excited very little.
Another problem associated with conventional acoustic transducers such as audio loudspeakers and the like arises from diffraction effects. For example, the edges of a conventional loudspeaker cabinet produces diffracted waves which interfere with the direct sound waves from the diaphragm of the speaker causing additional irregularities in the pressure response as a function of frequency. The severity of the effect depends upon the cabinet shape and the location of the diaphragm on the cabinet.